Method of fabricating a laminated metal member

ABSTRACT

A laminated, relatively massive metal member requiring a minimum of finish machining is made by cutting a number of thin metal laminae in the shape of the final member. The laminae are stacked into the shape of the member, and held therein by suitable restraining means. The laminae stack is then diffusion bonded together at an elevated temperature and pressure in a nonoxidizing environment.

i I United States Patent 1151 3,670,397

Lewis [4 June 20, 1972 [54] METHOD OF FABRICATING A 2,700,632 1/1955Ackerlind ..29/493 LANIINATED METAL MENIBER 2,998,646 9/1961 Hitz..29/497.5 3,133,346 5/1964 Allen ..29/497.5 [721 Invent G LewisPasadena 3,186,083 6/1965 Wright, Jr ...29/498 x [73] Assignee; NbflhAmerican Rockwell Corporation 3,207,503 9/ 1965 Clover, Jr. et al...29/493 3,345,735 10/1967 Nichols ..29/47l.l [22] 1970 3,393,445 7/1968Ulam ..29 497.5 [21] Appl. No.: 13,743

Primary Examiner-John F. Campbell Relaied Application Data AssistantExaminer-Richard Bernard Lazarus [63] Continuation-in-part of Ser. NO.604,270, Dec. 23, Lee Humphnes 1966, abandoned.

[57] ABSTRACT [52] U.S.Cl ..29/472.3,29/47l.1,29/493 A laminated,relatively massive metal member requiring a [51] hit. Cl minimum offinish machining is made y cutting a number of [58] Field of Search..29/472.3,497.5,493,247il thin metal laminae in the Shape of the finalmember The laminae are stacked into the shape of the member, and heldtherein by suitable restraining means. The laminae stack is [56]References Cited then diffusion bonded together at an elevatedtemperature UNITED STATES PATENTS and pressure in a non-oxidizingenvironment.

1,744,588 1/1930 Strauss "29/4723 4 Claims, 10 Drawing Figures SHEET 1BF 6 PATENTED-Jmo I972 F/E. Ea,

F/E'. Ec-

INVENTOR. JOSEPH C. LEW/5 PATENTEDaunzo 1972 3,670,397 sum 3 or eINVENTOR. J se-PH c. LEW/.5

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INVENTOR. J'O6E'PH c. LEW/S ATTQQNEY PATENTEDJum 1m SHEET 8 OF 6 OOO NWOGOO MN 3Q 0 9mm I, l I I l I I I I 7 W533 MUZWWSQRM 5 RM W m5 M 4 ma m wJ M/ 9 la 3 g S M S I h i ATTORNEY METHOD OF F ABRICATING A LAMINATEDMETAL MEMBER CROSS-REFERENCE TO RELATED APPLICATION This application isa continuation-in-part of the following:

Applicant: Joseph C. Lewis Serial No: 604,270

Filed: Dec. 23, I966, now abandoned,

For: Method of Fabricating A Laminated Metal Member The presentinvention relates to a method of fabricating a laminated metal member.

Large, complex, heavy metal fittings and the like are generallymanufactured either from castings or by forging a billet in a die havingapproximately the geometry of the final member. After forging, thebillet is machined into the final desired dimensions. With complex partshaving substantial mass non-uniformly distributed throughout the same,this frequently requires extensive precision machining. Initial costs offorgings are high and machining fittings of complex design frequentlyresults in scrap losses of as much as 80 percent. Also, unacceptableweight penalties, particularly for aircraft and spacecraft applications,may result because forged billets have less than optimum metallurgicalproperties due to insufficient reduction of the metal during forging orrolling. Moreover, the extreme loads and temperatures experienced duringsupersonic flight require the use of costly, high strength metals suchas titanium, hot-worked die steels, or maraging steels. In addition toscrap losses resulting from machining of costly metals, the investmentin expensive forging dies is a significant economic factor when lowproduction numbers are involved. Further, machining of complex shapes isexpensive and time-consuming. Another limitation inherent in forging isthat, since work is performed on a billet from the outside rather thanfrom inside, structures with interior cavities cannot be fabricated. Asis known, hollow box-type structures can be as strong as a solidstructure and without the weight penalty.

Fittings produced by the initial forging of billets are characterized byanisotropic properties (extreme directionality) which tend to reduce themechanical strength of the member and lead to failures under lesserloads. If compensation is made for poor metallurgical properties byincreasing the weight of the members, the economic penalties of lowstrength-to-weight ratios are very severe in aircraft and spacecraft.Further, machining itself can introduce stresses and other weaknessesinto. a workpiece which may lead to failure thereof under heavy loadsand temperatures. The difficulties of machining complex shapes fromheavy forged billets additionally impose undesirable constraints on thedesign of such members.

The principal object of the present invention, therefore, is to providean improved method of fabricating a metallic member consisting of asolid unitary mass.

Another object is to provide an improved method of fabricating acomplex-shaped, relatively massive metal member which requires a minimumof final machining and produces smooth, bright surfaces.

Another object is to provide a method of fabricating a high qualitystructural fitting which is stronger than a similar structural memberprepared by casting or forging and machining,

and in which scrap losses are greatly reduced.

Still another object is to provide such a method which will producefittings having greater strength than similar forged fittings.

Still another object is to provide a method of making a laminatedfitting having improved strength and metallurgical uniformity whereinsubstantial savings in cost and production time can be achieved.

A further object is to provide such a method which permits versatilityin the design of structural fittings and permits design changes withoutthe capital investment losses in expensive dies.

Yet another object is to provide a method of fabricating fittings havinghollow interior portions.

Other objects and advantages of the present invention will become clearfrom the following detailed description taken together with theaccompanying drawings and the attached claims.

In the drawings,

FIG. 1 is an overall view of a laminated trunnion bearing cap preparedaccording to the present invention;

FIG. 2 shows an individual lamina in various stages of fabrication;

FIG. 3 is a grouping of individual laminae of different designs utilizedin making the assembly of FIG. 1;

FIG. 4 shows the laminae of FIG. 3 in a complete assembly;

FIG. 5 shows the stack of laminae and associated equipment utilized inthe bonding operation;

FIG. 6 shows the assembly of the laminae stack and of associatedequipment in preparation for bonding; and

FIG. 7 is a graph comparing physical properties of forged and laminatedbillets.

The present invention provides a method of fabricating a laminated,complex-shaped, metal member requiring a minimum of final machining,which comprises providing a plurality of shaped, relatively thin, metallaminae designed to be arranged into said member, stacking the laminaeto form the shape of said member, and diffusion bonding the stack oflaminae together at an elevated temperature and pressure in an inertenvironment.

The stack of thin sheet material is bonded into a precision laminatedform of uniform properties having the exact configuration of the finalpart, which requires minimal, if any, final machining. The sheet laminaehave better metallurgical properties, which result in a fitting ofimproved strength and metallurgical uniformity over fittings machinedfrom large billets, castings and heavy forgings. For example, a memberproduced by practice of the present invention has on test shown anendurance limit of over twice that of a similar forged member. Byalternating the grain direction of each laminae, fittings havingisotropic properties can be produced from materials that exhibit extremedirectional properties in wrought form. Material losses are greatlyminimized, which contributes to the economic effectiveness of theprocess. Further, design flexibility is provided, and favorablestrengthto-weight ratios are obtained.

Enclosing the laminae stack in a restraining means produces a fittinghaving smooth and bright surfaces, equivalent to those of therestraining metal. This important benefit results from plastic flow atthe edges of the laminae under heat and pressure. Therefore, arestraining means serves not only to maintain alignment and preventdistortion during diffusion bonding, but also to polish the surfaces ofthe member.

The present method is further readily adapted to making hollow box-likemembers of high strength-to-weight ratios. This may be accomplished byutilizing interior laminae with aligned, cut-out portions and solidlaminae for exterior surfaces. The present method is also useful forsolid workpieces, particularly those having a thickness greater thanmaximum thickness of commercially available rolled plate.

Diffusion bonding is known to the art for joining members, for example,tubular or rib members, in point-to-point contact (usually notsurface-to-surface). It is generally considered as an alternativejoining method, superior in a number of regards to brazing or welding.Diffusion bonding is characterized by the formation of a metal-to-metalbond between contacting surfaces at suitable pressures and attemperatures below the melting point of the metal. Bonds which approachthe strength of the parent metal can be obtained. In certain cases athin interleaf material, or eutectic former, is provided, and in otherforms of diffusion bonding no interleaf material is utilized.

The mechanism of diffusion bonding is believed to involve plasticdeformation of the metal, followed by surpassing of the compressiveyield strengths. This characteristic of plastic deformation underpressure and temperature may be taken advantage of in the presentinvention to produce a finished part having smooth surfaces withoutfurther work. Inserting the laminae stack in a restraining die ofconforming shape mmxd m causes creep flow at the ends of the laminae toproceed along the edges of the stack. This has a smoothing and polishingeffect on edges which otherwise might be relatively rough and dull.

A number of metals may be joined by diffusion bonding, and illustrativeof these are aluminum, stainless steel, titanium, nickel, tantalum,molybdenum, zirconium, and columbium, and alloys of the foregoing. Forfurther information on diffusion bonding, reference is made to suchrepresentative US. Pat. Nos. as 3,145,466; 3,180,022; 3,044,160;2,850,798; 3,158,732; 3,170,234; and 3,242,565.

The present invention will be particularly illustrated with reference tothe accompanying drawings which disclose the preparation of a trunnionbearing cap for a supersonic aircraft from a 6 Al 4 V titanium alloy.This fitting had previously been made by forging the titanium alloy in asuitable die, and then machining the resulting billet. Referring to FIG.1, it is seen that the bearing cap 2 is massive and generally arcuate;it is about 12.25 in. long by 3.25 in. high. (Two such bearing caps arejoined to enclose the bearing.) The fitting has clevis bores 4 and slots6 at its extremities and elongated slots 8 extending through its body,in order to reduce its weight, but is essentially a solid unitary masswith non-uniform mass distribution, since it is wider and thicker ateach end than in its mid-portion. It is apparent that a hollow, box-typestructure of the same or other designs may be made, when required, bynot slotting the top and bottom laminae. The method of making thisbearing cap to semi-finish dimensions requiring minimum machining willbe described with reference to FIGS. 2-6.

FIG. 2ac illustrates the sequence for fabricating a lamina 12. Thelamina is made from 6 Al 4 V Ti plate stock approximately three-eighthsin. thick; it is friction sawed oversize, drilled, and profiled-to thefinish dimensions of the part except for an undercut. The slots 8 areobtained by use of a 0.312 in. mill cutter, and small alignment bores 10are drilled. As seen in FIG. 2a, sharp edges are not removed by sawingbut by machining (FIG. 2b and 2c). The lamina is lapped flat andparallel to a surface finish of 20 micro inches RMS.

In FIG. 3 are shown, next to lamina l2, laminae l4 and 16 of othershapes, which are required to make the configuration of FIG. 1; theseare made by the procedure described with respect to FIG. 2. Lamina I2 isan outer lamina, lamina 14 a shortened spacer, and lamina 16 an interiorlamina which, together with lamina 14, serves to define extremity slots6. Lamina 14 and 16 have longitudinal slots 8a and 8b and bores 10a,10b, and 4b, respectively, which align with the corresponding slots 8and bores 10 and 4 of lamina 12. Eight such laminae (two laminae 12, twolaminae l4, and four laminae 16) are utilized to form the bearing cap.

FIG. 4 shows the assembled laminae configuration 17, prior to bonding,including the positions of laminae l2, l4 and 16. This configuration issupported by means of spacer blocks 18 of Type 321 stainless steel whichare inserted in slots 8. These blocks serve to prevent warpage of theextremities of the assembly under high loads during diffusion bonding.Stainless steel pins 20 pass through clevis bores 4 and spacer blocks 18to maintain alignment of the laminae. Pins 22 pass through the smalleralignment bores 10. The stainless steel pins and spacer blocks arecoated with a step-off compound which will prevent bonding to thetitanium laminae. For example, aluminum oxide may be flame-sprayedthereon for this purpose. Likewise, the stainless steel members shown inFIG. 5 are coated with aluminum oxide.

FIG. 5 shows the associated apparatus employed in the diffusion bondingoperation. A rectangular retort 24 is fabricated from Type 321 stainlesssteel sheet of 0.035 in. thickness. A three-eighths in. diameterstainless steel purge tube, 36 in. long (not shown) is welded onto aport 26 of the retort.

A tooling stack consisting of three Type 321 stainless steel plates 28with their centers 30 cut out to conform with the shape of and receivethe stack of laminae is made to fit inside retort 24. The tooling stackprovides mass for uniform heating and prevents the retort fromcollapsing around the laminated billet during diffusion bonding undervacuum. The tooling stack also aligns and constrains the laminae stackin the manner of a constraining die. Stainless steel alignment pins 32are provided to fit the alignment holes 34 in plates 28. A stainlesssteel pressure plate 36, one-fourth in. thick, is cut to the shape ofthe laminae with one-sixteenth in. overlap on all edges.

Two other stainless steel plates 38, one-fourth in. thick, are alsoprovided, one to fit the bottom of retort 24 and the other as a topcover plate for the entire assembly.

Turning to FIG. 6, the arrangement of the tooling stack and the laminaestack in the retort is seen. FIG. 6a illustrates tooling plates 28disposed in retort 24, the plates being aligned by means of thestainless steel pins 32. Prior to lay-up in the retort, the stainlesssteel members are cleaned, for example, with an alkaline degreasingagent, followed by water and organic e. g., methylethylketone) rinses.The laminae stack 17 is then positioned in the tooling stack as shown inFIG. 6b. The flame-sprayed stainless steel pressure plate 36 shownoutside the retort in FIG. 6b is put into position over laminae stack 17by tack welding stainless steel tabs between the pressure plate and thetooling stack. The laminae stack is maintained at a height approximatelyone-eighth in. above the height of the tooling stack for all diffusionbonding operations so that the applied load is transmitted only throughthe stack.

Following lay-up, a top cover plate 38 (FIG. 5) is welded onto theretort. The closed retort is then leak tested under full vacuum, using ahelium leak detector to locate any leaks. All leaks are closed bywelding. The retort is hot purged at a temperature of about 600 F. in anelectric furnace by alternately flooding with dry argon and evacuatingto a full vacuum. The vacuum is maintained during all subsequent heatingand cooling periods.

After hot purging, the retort is removed from the electric furnace andplaced in a hydraulic press of 500 tons capacity which is equipped withceramic heating platens. Diffusion bonding is accomplished by heatingunder unidirectional compressive force for a sufficient period toachieve such bonding. The time-temperature-pressure relationships toachieve satisfactory bonding are coordinated and will vary with themetallurgical characteristics of the metal being bonded and thethickness of the laminae stack. Such bonded parameters may be determinedby those skilled in the art for different laminating applications fromavailable information concerning the diffusion bonding of particularmetals and their alloys. Generally, for the fabrication of the abovedescribed titanium member, it is found that coordinatedtime-temperature-pressure relationships of about 3% 4% hours,1,500-1,650 F. and 250-800 psi are satisfactory.

After the bonding operation is completed, the retort is removed from thepress and the diffusion bonded laminated stack removed from the retort.Any necessary finish machining, such as that required to achieve thedimensions of the trunnion bearing cap shown in FIG. 1 from theconfiguration of FIG. 4, is then performed. The pins 20 and 22 andspacer blocks 18 utilized to maintain alignment of the laminae stack 17(FIG. 4) are readily removed after the diffusion bonding step, sincebonding does not occur between the aluminum oxide-coated steel and thetitanium.

The laminated fitting generally has properties closely approaching thoseof the parent metal, particularly with regard to small grain sizeachieved in thin rolled sheets and not obtainable in massive castings orforgings. The bearing caps are tested by placing the fittings in a testfixture so that a contoured plunger can apply a bearing load against theback of the cap. This bearing load is reacted by pins through theclevises, placing the bearing cap in hoop tension. The clevises undergobearing failure at about 350,000 pounds load.

Additional data comparing the metallurgical properties of similarmembers fabricated by the present invention and by forging will now bepresented. FIG. 7 displays the notched fatigue data or endurance limits(i.e., the maximum stress load which can be maintained without failureunder repeated cycling) of forged and of laminated billets of6Al-6V-2Sn-Ti alloy prepared by the present method. It is seen that theendurance limit of the laminated billet is over twice that of the forged(57,000 psi vs. 25,000 psi).

The following table presents a comparison of the notch properties of a 3X 3 X 6 inches laminated and forged billet of the foregoing titaniumalloy. The better notch properties and notch strength to yield strengthratios (although not yield strengths) of the laminated billet areclearly apparent.

TABLE I Notch Yield Strength Strength Notch Strength] Material (KS1)(KS1) Yield Strength Forged 72.9 160.6 0.45

Billet 74.7 0.47 Laminated 165.0 154.4 1.07

Billet I claim:

1. A method of fabricating a laminated, complex-shaped, metal memberrequiring a minimum of final machining which comprises fabricating aplurality of shaped, relatively thin metal laminae designed to bearranged into said member, stacking said laminae substantially parallelto each other to form the shape of said member, placing the resultinglaminae stack within an enclosed metal retort having disposed therein arestraining die shaped to receive and to conform with the shape of saidlaminae stack, drawing a vacuum on the resulting assembly, and thenapplying suitable heat and pressure to said assembly in order todiffusion bond said laminae together.

2. The method of claim 1 wherein the said laminae are of titanium, andsaid diffusion bonding is conducted under coordinatedtime-temperature-pressure conditions of about 3% 4% hours, 1,500-1,650F, and 250-800 psi.

3. The method of claim 1 wherein said laminae are of titanium and saidretort and said restraining die are of stainless steel, and whereinsurfaces of said stainless steel contacting said titanium are treated toprevent bonding therewith.

4. The method of claim 1 wherein said heat and pressure are applied byplacing said retort assembly between heating platens and then applyingunidirectional compressive force to the resulting assembly.

1. A method of fabricating a laminated, complex-shaped, metal memberrequiring a minimum of final machining which comprises fabricating aplurality of shaped, relatively thin metal laminae designed to bearranged into said member, stacking said laminae substantially parallelto each other to form the shape of said member, placing the resultinglaminae stack within an enclosed metal retort having disposed therein arestraining die shaped to receive and to conform with the shape of saidlaminae stack, drawing a vacuum on the resulting assembly, and thenapplying suitable heat and pressure to said assembly in order todiffusion bond said laminae together.
 2. The method of claim 1 whereinthe said laminae are of titanium, and said diffusion bonding isconducted under coordinated timE-temperature-pressure conditions ofabout 3 1/2 - 4 1/2 hours, 1,500*-1,650* F, and 250-800 psi.
 3. Themethod of claim 1 wherein said laminae are of titanium and said retortand said restraining die are of stainless steel, and wherein surfaces ofsaid stainless steel contacting said titanium are treated to preventbonding therewith.
 4. The method of claim 1 wherein said heat andpressure are applied by placing said retort assembly between heatingplatens and then applying unidirectional compressive force to theresulting assembly.